A group of 9 piles with 3 piles in a row was driven into a soft clay extending from ground level to a great depth. The diameter and the length of the piles were 30 cm and 10m respectively. The unconfmed compressive strength of the clay is 70 kPa. If the piles were placed 90 cm center to center,

Question

A group of 9 piles with 3 piles in a row was driven into a soft clay extending from ground level to a great depth. The diameter and the length of the piles were 30 cm and 10m respectively. The unconfmed compressive strength of the clay is 70 kPa. If the piles were placed 90 cm center to center, compute the allowable load on the pile group on the basis of a shear failure criterion for a factor of safety of 2.5.

Accepted Answer

The allowable load on the group is to be calculated for two conditions: (a) block failure and (b) individual pile failure. The least of the two gives the allowable load on the group.

(a) Block failure (Fig. 15.27). Use Eq. (15.70),

[latex]Q_{g u}=c N_{c} A_{g}+P_{g} L \bar{c}[/latex] (15.70)

[latex]\begin{aligned}&Q_{g u}=c N_{c} A_{g}+P_{g} L \bar{c} \quad ext { where } \quad N_{c}=9, c=\bar{c}=70 / 2=35 kN / m ^{2} \&A_{g}=2.1 imes 2.1=4.4 m ^{2}, \quad P_{g}=4 imes 2.1=8.4 m , L=10 m \&Q_{g u}=35 imes 9 imes 4.4+8.4 imes 10 imes 35=4326 kN , \quad Q_{a}=\frac{4326}{2.5}=1730 kN\end{aligned}[/latex]

(b) Individual pile failure

[latex]Q_{u}=Q_{b}+Q_{f}=q_{b} A_{h}+\alpha \bar{c} A_{s} \cdot ext { Assume } \alpha=1[/latex].

Now, [latex]q_{b}=c N_{c}=35 imes 9=315 kN / m ^{2}, A_{b}=0.07 m ^{2}[/latex],

[latex]A_{s}=3.14 imes 0.3 imes 10=9.42 m ^{2}[/latex]

Substituting, [latex]Q_{u}=315 imes 0.07+1 imes 35 imes 9.42=352 kN[/latex]

[latex]Q_{g u}=n Q_{u}=9 imes 352=3168 kN , \quad Q_{a}=\frac{3168}{2.5}=1267 kN[/latex]

The allowable load is 1267 kN.

Question 15.25: A group of 9 piles with 3 piles in a row was driven into a s...

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